1. Field of the Invention
This invention relates to a light reception signal circuit for a photoelectric switch intended for detecting the presence of an object by means of a beam of infrared light or the like.
2. Description of the Prior Art
A photoelectric switch detects the presence or absence of an object according to whether or not a beam of light such as, for example, an infrared beam is intercepted by the object. A photoelectric switch may be a transmission type as shown in FIG. 6 or a reflection type as shown in FIG. 7.
The transmission type of photoelectric switch shown in FIG. 6 includes a light emitting part 11 and a light receiving part 12 disposed opposite each other. An object 13 (a moving member, for example) may be interposed therebetween. A light beam 14 such as an infrared beam is emitted from the light emitting part 11 toward the light receiving part 12. In the reflection type device of FIG. 7, a light emitting/receiving part 15 functions as both a light emitting part and a light receiving part at the same time. A reflecting plate 16 is disposed opposite to the light emitting/receiving part 15. An object 13 may be interposed therebetween. The light beam 14 from the light emitting/receiving part 15 is reflected from the reflecting plate 16 and returned to the light emitting/receiving part 15.
Both of the above types generate an electrical signal responsive to the amount of the light beam 14 received. If the light beam 14 is intercepted by the object 13, a change (reduction) takes place in the electrical signal. Thus the presence or absence of the object 13 at a predetermined position is detected.
FIG. 8 represents a prior art light reception signal circuit for detecting whether or not the light 14 is intercepted. In the drawing, a reference character OP.sub.1 denotes an operational amplifier used as a photoelectric conversion circuit. A photoelectric conversion element, or a photodiode PD is connected, in the direction illustrated, between the plus (+) input and the minus (-) input of operational amplifier OP.sub.1. A constant voltage V.sub.ref is impressed on the plus (+) input of operational amplifier OP.sub.1. A variable resistor R.sub.1 is connected between output and the minus (-) input of operational amplifier OP.sub.1.
A reference character OP.sub.2 denotes an operational amplifier used as a voltage amplifier circuit. The constant voltage V.sub.ref is applied to the plus (+) input of operational amplifier OP.sub.2. An output voltage V.sub.1 of the operational amplifier OP.sub.1 is applied to the minus (-) input of operational amplifier OP.sub.2 through an input resistance R.sub.2. A feedback resistance R.sub.3 is connected between the output and the minus (-) input of operational amplifier OP.sub.2.
A reference character OP.sub.3 denotes a voltage comparator (hereinafter called a comparator). An output voltage V.sub.2 of the operational amplifier OP.sub.2 is applied to the plus (+) input of comparator OP.sub.3. The voltage V.sub.2 is compared with a standard voltage V.sub.s to produce an output voltage V.sub.3.
In the aforementioned construction, when the light beam 14 is not intercepted by the object 13, the voltage V.sub.2 is larger than the standard voltage V.sub.s. The output voltage V.sub.3 of the comparator OP.sub.3 is at the "H" level. On the other hand, if the light beam 14 is intercepted by the object 13, the output voltage V.sub.2 is lower than the standard voltage V.sub.s, and the output voltage V.sub.3 of the comparator OP.sub.3 decreases to the "L" level. That is, the output voltage V.sub.3 of the comparator OP.sub.3 generates a signal at the "H" or "L" level according to the position of the object 13. In this way, the presence or absence of the object 13 can be detected.
If a measuring range (distance) l in FIG. 6 and FIG. 7 is reduced, the quantity of received light increases. If object 13 is small, or translucent, or is displaced from the center of the light beam 14, enough light may pass the object 13 to maintain the voltage V.sub.2 above the standard voltage V.sub.s. In this condition the presence of the object 13 cannot be detected.
To prevent such erroneous operation due to a change in the distance l, the photocurrent produced by photodiode PD is controlled by adjustment of the variable resistor R.sub.1. In this way the output voltage V.sub.2 applied to the comparator OP.sub.3 is adjusted to an appropriate voltage, such as, for example, V.sub.2 =2 V.sub.s.
The aforementioned control is effected by a user by manually adjusting the variable resistor R.sub.1 whenever the distance l is changed or following initial installation. When installed outdoors, the quantity of light may decrease due to mist, dust and the like. Such a decrease in light may likewise be compensated for by adjustment of the variable resistor R.sub.1.